1. Field of the Invention
The present invention relates to an electronic flash device capable of emitting flash light having an intensity that is maintained constant over a predetermined period of time after initiation thereof. Such a flash device may be used to illuminate an object to be photographed in advance of actual exposure to determine a proper exposure condition for the subsequent flash photograph. The flash device may also be used with a focal plane shutter camera, or a slit shutter camera to illuminate the photographic object while the slit travels in front of the film.
2. Description of the Prior Art
The electronic flash device proposed by Japanese Laid-Open Patent No. 47-27527 is basically composed of a Xenon flash tube, a switching element serially connected with the Xenon tube, a main capacitor connected across the serially connected Xenon tube and switching element for storing electric energy for flash firing, and another auxiliary capacitor connected in parallel with the Xenon tube. During the light emission from the Xenon tube, the switching element is periodically and alternately turned on and off by a control means to intermittently supply electric energy from the main capacitor to the Xenon tube. While the switching element is being turned off to interrupt the energy supply, the auxiliary capacitor is discharged through the Xenon tube to energize the latter, so that the intensity of light emitted from the Xenon tube is maintained substantially constant over a predetermined period of time.
In the circuit construction of such an electronic flash device, however, the switching element must be adapted to have a finite non-zero internal resistance, at its turned-on condition to obtain the desired light emission characteristics. This is because the voltage applied across the end terminals of the Xenon tube must be lower than the voltage across the main capacitor in order to fire the Xenon tube at a constant light intensity over a predetermined period of time using the energy stored in the main capacitor. With the above electronic flash device construction, it is necessary for the switching element connected in series with the Xenon tube to include a resistance component so as to provide the Xenon tube with a voltage lower than that of the main capacitor. In such prior art electronic flash device, energy loss inevitably occurs in the switching element that is not contributable to the light emission.